The Return of the King, LoTR Appendix B, The Tale of Years: The Third Age

[It] was not until 2000 that [the Nazgûl]... laid siege to Minas Ithil. This they took in 2002, and captured the palantír of the tower.

The Return of the King, LoTR Appendix A, Annals of the Kings and Rulers: The Realms in Exile: The Southern Line: Heirs of Anárion

At this time probably the [palantír2] of Minas Ithil was captured and so came to the hands of Sauron.

The Peoples of Middle-Earth, HoME Vol 12, Part 1, Ch 7, The Heirs of Elendil: The Southern Line of Gondor: The Anárioni

Two things contributed then to the neglect of the Stones, and their passing out of the general memory of the people. The first was ignorance of what had happened to the Ithil-stone: it was reasonably assumed that it was destroyed by the defenders before Minas Ithil was captured and sacked; but it was clearly possible that it had been seized and had come into the possession of Sauron, and some of the wiser and more farseeing may have considered this. It would appear that they did so, and realized that the Stone would be of little use to him for the damage of Gondor, unless it made contact with another Stone that was in accord with it. It was for this reason, it may be supposed, that the Anor-stone, about which all the records of the Stewards are silent until the War of the Ring, was kept as a closely-guarded secret, accessible only to the Ruling Stewards and never by them used (it seems) until Denethor II.

The second reason was the decay of Gondor, and the waning of interest in or knowledge of ancient history among all but a few....

1In the entry in the Tale of Years for 2002, and also in Appendix A (I, iv), it is stated as a fact that the palantír was captured in the fall of Minas Ithil; but my father noted that these annals were made after the War of the Ring, and that the statement, however certain, was a deduction.

Unfinished Tales, Part 4, Ch 3, The Palantíri: Notes, Note 4

2This text is from one of Tolkien's early drafts. For the sake of clarity, earlier versions of proper names used by Tolkien in this draft have been replaced with the versions in use in the canon sources. All substitutions are marked with brackets.